Vela as the source of Galactic cosmic rays above 100 TeV

Abstract

We model the contribution of the nearest young supernova remannt Vela to the local cosmic ray flux taking into account both the influence of the Local Superbubble and the effect of anisotropic diffusion. The dominant contribution of this source in the energy region around the cosmic ray knee can naturally explain the observed fluxes of individual groups of nuclei and their total flux. Adding the CR flux from a 2-3 Myr old local CR source suggested earlier, the CR spectra in the whole energy range between 200 GeV and the transition to extragalactic CRs are described well by the combined fluxes from these two local Galactic sources.Keywords: High energy cosmic rays, Vela supernova remnant, Local Superbubble, Galactic magnetic field.The measured energy spectrum of cosmic rays (CRs) extends smoothly over more than 11 decades as a nearly featureless power law, I(E) ∝ E −β . One of its most prominent features is the knee, a break in the all-particle energy spectrum at the energy E k 4 PeV, which was discovered by Kulikov and Khristiansen in the data of the MSU experiment already in 1958 [1]. The second knee corresponds to a change in the spectral slope of the all-particle energy spectrum at 5 × 10 17 eV where the slope hardens by ∆β 0.2. There is a general consensus that the knee in the total CR spectrum at E k 4 PeV coincides with a suppression of the primary proton and/or helium flux, and that the composition becomes increasingly heavier in the energy range between the knee and 10 17 eV [2][3][4][5].Explanations for the origin of the knee fall in two main categories, connecting it either with a change in the propagation or the injection of CRs. In the first case, the knee energy may either corresponds to the rigidity at which the CR Larmor radius R L is of the order of the coherence length l c of the turbulent magnetic field in the Galactic disk [6,7]. Alternatively, the knee corresponds to a transition between the dominance of pitch angle scattering to Hall diffusion or drift along the regular field [8][9][10]. In both cases, the energy dependence of the confinement time changes which in turn induces a steepening of the CR spectrum [6][7][8][9][10][11]. In the second class of models, the knee is connected to properties in the injection spectrum of the Galactic CR sources. For instance, the knee might correspond to the maximal rigidity to which CRs can be accelerated by the population of Galactic CR sources dominating the CR flux below PeV [12][13][14]. Alternatively, the knee may be caused by a break in the source CR energy spectrum at this rigidity [15,16]. A variant of this model is the suggestion that the spectrum below the knee is dominated by a single, nearby source and that the knee correspond to the maximal energy of this specific source [17,18]. All these models lead to a sequence of knees at ZE k , a behaviour first suggested by Peters [19].In the isotropic diffusion approximation one defines a scalar diffusion coefficient which depends on energy as D(E) = D 0 (E/E 0 ) δ . Measurements of the Boron and Carb...

The origin of Galactic cosmic rays: Challenges to the standard paradigm

“…It is possibly for this reason that several alternative ideas to the SNR paradigm, proposed years ago to explain the origin of CRs, have been recently revived. The list includes the acceleration of particles at the Galactic centre (see 12 for an early reference, 255 for a recent one), at stellar winds (see 280 for an early reference, 281 for a recent one), in superbubbles (see 282,283 for early references, [259][260][261] for recent ones), or the explanation of at least part of the CR spectrum with a single (or very few) sources (see 284 for an early reference, 285 for a recent one). Finally, other classes of sources (such as X-ray binaries 286,287 or pulsar wind nebulae 288 ) have been considered, and attempts to develop truly heterodox scenarios were published, as we briefly discussed in Section 3.1.5.…”

Cosmic ray models

“…Such an excess is avoided, if one takes into account that the Sun is located inside the Local Bubble: In Ref. [154], the Local Bubble was modelled as a cylinder with base radius R = 100 pc, a bubble wall of thickness to w = 3 pc and B sh = 12µG, and B out = 1µG outside the bubble. The Sun was assumed to be at the centre of the Local Bubble, while Vela was set at the distance 270 pc from the Sun along the magnetic field.…”

“…Left panel: The CR flux of protons around Vela, in the wall and at Earth as function of energy E. Right panel: The all-particle flux at Earth from a 2-3Ṁyr old SN (purple) and Vela (red) together with experimental data.Adapted from[154].…”

Vela as the Source of Galactic Cosmic Rays above 100 TeV

“…The model used for the local magnetic field is very similar to the one used in [52], we apply an exponential damping of the bubble magnetic field at z bub ≈ ±3 pc to ensure the decaying of the bubble magnetic field from the top and the bottom. The strength of the regular magnetic field depends only on the radius and is set to B in = 0.1µG inside the bubble, B sh = 8 − 12µG in the wall, and B out = 1 − 3µG outside the bubble.…”

“…In order to compute the flux, we injected 30.000 protons per energy at the position of Vela and propagated them for 12.000 yr. We calculated the CR density n(E) in three regions of interest averaging the CR densities between 8 to 12 kyr: Around the source, on the bubble wall, and inside the bubble. The CR flux F(E) = c/(4π)n(E) was then computed from the CR densities in the considered volumes, as defined in [52] For energies below 100 TeV we deduced the flux from earlier times and higher energies using the scaling relation for the flux inside the bubble :…”